The present invention relates to optical films useful, e.g., as polarizers and/or mirrors.
Light-reflecting devices based upon multiple polymeric layers are known. Examples of such devices include polarizers made of alternating polymeric layers in which the layers have different refractive indices.
The optical properties and design considerations of birefringent optical films described herein allow the construction of multilayer stacks for which the Brewster angle (the angle at which reflectance of p-polarized light goes to zero) is very large or is nonexistant. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p-polarized light decreases slowly with angle of incidence, are independent of angle of incidence, or increase with angle of incidence away from the normal. As a result, multilayer films having high reflectivity (for both s and p polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers) over a wide bandwidth, can be achieved.
Briefly, in one aspect the present invention provides a multilayered polymer film comprising layers of a crystalline or semi-crystalline naphthalene dicarboxylic acid polyester, for example a 2,6-polyethylene naphthalate (xe2x80x9cPENxe2x80x9d) or a copolymer derived from ethylene glycol, naphthalene dicarboxylic acid and some other acids such as terephthalate (xe2x80x9cco-PENxe2x80x9d), with a positive stress optical coefficient, i.e. upon stretching its index of refraction in the stretch direction increases, having an average thickness of not more than 0.5 microns; and layers of a selected second polymer, for example a polyethylene terephthalate (xe2x80x9cPETxe2x80x9d) or a co-PEN, having an average thickness of not more than 0.5 microns. Preferably, after stretching of the films of this invention in at least one direction, the layers of said naphthalene dicarboxylic acid polyester have a higher index of refraction associated with at least one in-plane axis than the layers of the second polymer. The film of this invention can be used to prepare multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.
In another aspect, the present invention provides a multilayered polymer film comprising layers of a crystalline or semi-crystalline polyester, for example a PET, having an average thickness of not more than 0.5 microns: and layers of a selected second polymer, for example a polyester or a polystyrene, having an average thickness of not more than 0.5 microns: wherein said film has been stretched in at least one direction to at least twice that direction""s unstretched dimension. The film of this invention can be used to prepare multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.